The invention relates to a new class of mesoporous (nickel/platinum)-yttria-zirconia materials, denoted meso-(Ni/Pt)YZ, which have utility as electrode materials in solid oxide fuel cells (SOFCs). They are synthesized by aqueous co-assembly of glycometallates and metal complexes with a surfactant template.
The solid-oxide fuel cell (SOFC) has been a leading candidate for both stationary and mobile power generation for the past 10 years. This is due to its all solid-state configuration, which eliminates moving parts and corrosive liquids, a high energy conversion efficiency not subject to the Carnot cycle, low emission of pollutants, and multi-fuel capability. SOFCs operate at elevated temperatures (600-1000xc2x0 C.) allowing them to process a multitude of fuels including methane and methanol, a key point considering the world is not yet on a hydrogen economy.
While SOFCs offer great potential as an alternative energy source, this technology has yet to become commercially viable mainly due to the nature of its active components, the anode and cathode. Since the inception of SOFCs, the electrodes have remained of the same basic form consisting, at least in part, as a dense ceramic phase. The anode has traditionally been a nickel/yttria-stabilized-zirconia (YSZ) cermet while the cathode material is usually a perovskite of the composition LaxSr1-xMnO3 or a platinum/YSZ composite. Electrodes have stringent requirements for use within a SOFC due to the high operating temperatures involved. These include stability in terms of chemical reactivity, phase, morphology, dimensionality, thermal expansion coefficient, catalytic activity, electronic and ionic conductivity and porosity. Existing electrode materials are intrinsically dense having zero intra-granular porosity at elevated temperatures and exhibiting low surface areas arising from inter-granular necking produced through sintering processes. Porosity is a singular attribute, which not only controls the transport of gaseous fuel/oxidant to reactive sites, but also the length of the triple-phase-boundary (TPB) where charge transfer occurs for an electronically/ionically conducting electrode. The TPB is defined as the interface where the electronically/ionically conductive electrode meets both the YSZ electrolyte and the gaseous fuel/oxidant. Both mass transport (gaseous diffusion, adsorption processes and surface diffusion) and charge transfer processes at the TPB limit the efficiency of SOFCs.
Several researchers have attempted to improve porosity and enlarge the TPB by manipulating the electrode microstructure through traditional solid state chemistry and material science techniques, which includes but is not limited to, the impregnation of YSZ with noble metal salts and chemical deposition of electrode materials on YSZ substrates. The common thread among these approaches involves enlarging the TPB by diminishing the dimensions of metal particles such as Ni, Pt, in relation to YSZ grains. In essence, these materials are nanoscale or microscale versions of the bulk cermet electrode materials having a comparatively wide pore size distribution with low thermal stability
The synthesis of mesoporous materials through surfactant-based self-assembly techniques has been an area of intense research since 1992; however, there are no adaptations of the technique that produce mesoporous yttria-zirconia analogues which are sufficiently thermally stable to function as SOFC materials. Most mesoporous transition metal oxide materials reported as being stable upon surfactant removal incorporate either phosphate or sulfate groups as stabilizers and should be regarded as oxo-sulfates or oxo-phosphates (Ying et al. U.S. Pat. No. 5,958,367). Further, these materials structurally collapse when these groups are removed upon heating to around 400xc2x0 C. Moreover, mesoporous yttria-zirconia versions have not been reported.
Accordingly, it is one of the purposes of this invention, among others, to produce SOFC electrode materials in which glycometallates and metal complexes are co-assembled with a surfactant template to produce a binary or ternary mesoporous-(metal)-yttria-stabilized-zirconia, meso-(M)YZ, which has uniform sized walls, high thermal stability (800xc2x0 C.) and electroactive catalytic sites, and high ionic/electronic conductivity.
The present invention is a method of producing a thermally stable mesoporous transition metal oxide composition. The method includes reacting a transition metal polyol-based gel with a surfactant in an aqueous environment under basic conditions. The transition metal polyol-based gel can be produced by dissolving a source of a transition metal in a polyol-based solvent with a high dielectric constant and coordinating ability to form a first solution; dissolving a source of a second metal in a second polyol-based solvent with a high dielectric constant and coordinating ability to form a second solution; and mixing these solutions to form the transition metal polyol-based gel.
The transition metals used in this invention to form the transition metal polyol-based gel can be any of the transition metals. A preferred source of the transition metal can be any transition metal alkoxide, transition metal glycolate or transition metal acetate. Preferred transition metal alkoxides are zirconium alkoxide, yttrium alkoxide, scandium alkoxide or rare earth alkoxides. The most preferred source of a transition metal alkoxide is zirconium ethoxide.
The second metal used in this invention to form the transition metal polyol-based gel can be any metal. A preferred source of the second metal can be any metal alkoxide, metal glycolate or metal acetate. Preferred metal alkoxides are yttrium alkoxide, scandium alkoxide, rare earth alkoxides, calcium alkoxide and magnesium alkoxide. The most preferred source of the second metal is yttrium acetate. Preferably, the resulting mesoporous transition metal oxide composition includes from about 1 to about 60 atomic % yttrium.
The polyol-based solvent is preferably ethylene glycol. The surfactant is preferably a neutral or cationic surfactant. The cationic surfactant can be a long-chain alkyl substituted ammonium salt. The preferred long-chain alkyl substituted ammonium salt is cetyltrimethyl ammonium bromide.
In one embodiment, the method can further include the addition of a metal precursor to the transition metal polyol-based gel. This would result in the formation of a mesoporous ternary transition metal oxide composition. The metal precursor can be a platinum precursor, a nickel precursor, a palladium precursor, a copper precursor, an iron precursor, a ruthenium precursor, a rhodium precursor or a cobalt precursor.
The method can further include the calcination of the mesoporous transition metal oxide composition to form a crystalline transition metal oxide composition. This crystalline transition metal oxide composition has uniform pore sizes from about 10xc3x85 to about 50xc3x85 in diameter. The mesoporous transition metal oxide composition remains stable upon removal of the surfactant templating agent without requiring use of oxyanion, hydride or halide stabilizers.
The present invention also provides a thermally stable solid oxide fuel cell (SOFC) electrode material that includes a metal-stabilized-zirconia. The surface area of the SOPC material is from about 150 m2/g to about 500 m2/g. The material preferably has uniform pore sizes from about 10 xc3x85 to about 50 xc3x85 in diameter.
The metal of the metal-stabilized-zirconia is compatible with zirconia, The compatible metal can be an alkaline earth metal or a transition metal. A preferred compatible metal is yttria. Preferably, the resulting mesoporous transition metal oxide composition includes from about 1 to about 60 atomic % ytria.
In one embodiment the thermally stable solid oxide fuel cell (SOFC) electrode material can further include a third metal. This third metal is a transition metal and is soluble with the metal-stablized-zirconia. This third metal can be titanium or niobium.
In another embodiment the thermally stable SOFC electrode material can further include a third metal incorporated as nanoclusters in the SOFC electrode material. The nanoclusters are uniformly dispersed throughout the SOFC electrode material. This third metal is a transition metal. Examples of this third metal are platinum, nickel, palladium, copper, iron, ruthenium, rhodium or cobalt.
The present invention provides a thermally stable mesoporous transition metal oxide composition that maintains its structural integrity in the temperature range of about 400-800xc2x0 C. without requiring the use of oxyanion, hydride or halide stabilizers. Upon calcination this composition provides a thermally stable SOFC electrode material that can include yttria-stabilized-zirconium. The surface area of this SOFC material is the highest yet observed for any form of yttria-stabilized-zirconia. By this self-assembly method of making SOFC electrode materials, a single phase material with a homogenous distribution of elemental components is created having intra-granular porosity, thereby allowing for a high TPB region within a single particle with much improved gas permeability/mass transport qualities. These characteristics greatly improve SOFC efficiency and lower operating temperatures to below 600xc2x0 C. Moreover, the meso-(M)YZ provided by this invention is the first example of a thermally stable mesoporous transition metal oxide, produced from templating with a common ionic surfactant, and that maintains its structural integrity in the temperature range 400-800xc2x0 C. More importantly, meso-MYZ is the first example of a nesostructure intentionally designed to be ionically conductive and electro-catalytically active for use as electrode materials in SOFCs. These and other advantages of the present invention will be appreciated from the detailed description and examples that are set forth herein. The detailed description and examples enhance the understanding of the invention, but are not intended to limit the scope of the invention.